We just don't let people join until they are 18. There is no way we can afford to have people undergo the training needed for child protection. I believe there are also problems with e.g. training facitilties, mixed trips etc.
In short, easier to avoid.
Totally agree with the easier to leave alone argument. I feel saddened though that we have a guy who wants to go caving, is really interested in Geology, looking to study at Uni in France for the caving and yet we can't take him underground. I've pointed him in the direction of the Scouts.